1. Field
The present invention relates generally to computer security and, more specifically, to synchronization of cryptographic processing between two entities in a processing system.
2. Description
In some digital content transmission and reception systems, a transmitter sends digital content, such as audio/video data, over a fast, high bandwidth, unidirectional connection to a receiver, which renders the content for perception by a user. This connection often is noisy; that is, communication of data may include some errors in transmission. Some of these systems also have a slow, very low bandwidth back channel for communicating from the receiver to the transmitter.
In some of these systems, the transmitter encrypts the content prior to communication of the content to the receiver in order to protect the content from unauthorized access during transmission. The receiver then decrypts the received encrypted content and renders it. Systems typically employ a stream cipher for encryption and decryption of the content. When the content is audio/video data (such as a television program or film), each frame is typically encrypted with a unique key stream, and the transmitter and the receiver must continuously agree on the current position of the encrypted video stream being sent from the transmitter to the receiver. A sync pulse is typically sent at the transmission of a frame boundary of video data. When the transmitter and receiver get out of synchronization due to missing a sync pulse, as commonly occurs when the transmitter changes video sources (e.g., when the user changes TV channels and requests delivery of a different program) or when transmission errors occur, the receiver improperly decrypts the video data because it may be using a different key stream than the transmitter. The result may be display of a garbled video image called “video snow” for a period of time. The transmitter can abandon the out-of-sync video stream and initiate transmission of a new stream (in effect “re-synchronizing” the data flow), but will not do so until and unless the out-of-sync situation is detected.
One known solution to this problem includes checking every 128th video frame of the video stream at the transmitter and receiver. However, this approach may still result in the display of “video snow” for up to four or five seconds in a worst case situation, depending on the frame rate being used. This result is undesirable from a viewer standpoint. Thus, new techniques are needed.